1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method, and a base station, in which a radio terminal transmits uplink user data to a base station via an enhanced dedicated physical data channel and the base station transmits transmission rate control data for controlling a transmission rate of the uplink user data to the radio terminal.
2. Description of the Related Art
Heretofore, a radio communication system including a base station and a radio network controller has been known. In the radio communication system, the base station has a single or multiple cells, and a radio communication is performed between each of the cells and a plurality of radio terminals. The radio network controller controls a plurality of base stations, and assigns a radio resource to the plurality of the radio terminals. Note that such a technique (hereinafter referred to as first technique) is sometimes referred to as R99 (Release 99) or the like.
For the purpose of improving the throughput and shortening the delay time, and the like, there has recently been proposed a technique in which a base station assigns the radio resources for uplink user data transmitted from each of the radio terminals to the base station (network side). Note that such a technique (hereinafter referred to as a second technique) is sometimes referred to as the high speed uplink packet access (HSUPA), the enhanced uplink (EUL) or the like.
Each of the cells functions as a serving cell or as a non-serving cell. A transport block size (TBS) is determined based on the transmission rate (for example, a scheduling grant (SG)) of the uplink user data, and is controlled by transmission rate control data transmitted from the serving cell and the non-serving cell. The transmission rate control data includes an absolute grant (AG) for controlling an absolute value of the transmission rate and a relative grant (RG) for controlling a relative value of the transmission rate (for example, see 3GPP TS25.321 Ver. 7.5.0).
Here, the uplink user data is transmitted to the base station from the radio terminals via an enhanced dedicated physical data channel (E-DPDCH). The absolute grant (AG) is transmitted from the radio base station to the radio terminals via an E-DCH absolute grant channel (E-AGCH). The relative grant (RG) is transmitted from the radio base station to the radio terminals via an E-DCH relative grant channel (E-RGCH).
The serving cell transmits the absolute grant (AG) and the relative grant (RG) to the radio terminals. Meanwhile, the non-serving cell transmits, to the radio terminals, only the relative grant (RG) without transmitting the absolute grant (AG).
Meanwhile, the base station has an upper limit of radio resources (maximum radio resource) that can be assigned to the radio terminal. Here, the maximum radio resource is referred to as the sum of the transmission rates (maximum receiving transmission rate) that can be assigned to the radio terminal by the base station.
In the second technique described above, the base station can transmit the absolute grant (AG) or the relative grant (RG) in each transmission time interval (TTI). In other words, the transmission rate assigned to the radio terminal can be changed in each TTI.
Here, in a case where the sum of the transmission rates already assigned to the radio terminals in a certain TTI is larger than the maximum receiving transmission rate, a reception quality of uplink user data is deteriorated when the base station receives the uplink user data in the certain TTI.
Meanwhile, even when the sum of the transmission rates already assigned to the radio terminals in a certain TTI is larger than the maximum receiving transmission rate, the sum of the transmission rates already assigned to the radio terminals in a different TTI is not necessarily larger than the maximum receiving transmission rate.
Under the circumstances as described above, when the transmission rate to be assigned to the radio terminal is controlled in consideration of only one TTI in which the transmission rate already assigned to the radio terminal is larger than the maximum receiving transmission rate, radio resources (transmission rate) may not be efficiently assigned to the radio terminal in other TTIs.